The present invention relates generally to a superconducting magnet which is cooled by a cryocooler, and more particularly to such a magnet which maintains its superconductivity (i.e., it does not quench) for an extended period of time during cryocooler failure.
Superconducting magnets may be used for various purposes, such as to generate a uniform magnetic field as part of a magnetic resonance imaging (MRI) diagnostic system. MRI systems employing closed or open superconductive magnets are used in various fields such as medical diagnostics. Open magnets typically employ two spaced-apart toroidal-shaped superconducting coil assemblies, while closed magnets typically employ a single solenoidal-shaped superconducting coil assembly. The superconducting coil assembly includes the magnet cartridge which contains the superconducting wire.
Some superconducting magnets are cooled by a cryocooler coldhead (such as that of a conventional Gifford-McMahon cryocooler). If there is an electric power outage or if the cryocooler otherwise malfunctions (or even has its performance degrade over time), the superconducting magnet will heat up and quench (i.e., lose its superconductivity). Prior-art magnets include those with sensor systems which monitor cryocooler temperature and which send an alarm signal when there is a cryocooler failure from whatever cause (including an electric power failure). The thermal hold time of a superconducting magnet is the period of time from cryocooler failure to quench and is generally limited by the thermal mass of the magnet. The thermal hold time of prior-art, cryocooler-cooled, superconducting magnets is not long enough to wait out a lengthy power failure of several hours without quenching. If a substantial solid thermal mass (such as a lead thermal ballast) is added to the magnet, the weight and volume of this added mass can pose problems due to space and structural support limitations. Also, the design of prior art superconducting magnets does not allow replacement of the cryocooler, in single-cryocooler systems, without quenching. What is needed is a superconducting magnet with an extended thermal hold time so the magnet can remain superconductive and operational while the malfunctioning cryocooler is repaired or replaced or while electric power is being restored. Further, the extended thermal hold time feature must not add significant weight, volume, cost, or complexity to the magnet.